I think I'm having the exact same issue on my M. I'm scared to drive it, but I'm trying to bring it over to the garage so we'll see if anything blows on the way there...Hi everyone, I've got yet another cooling-system-related question on my 2004 330Ci.
A month or so ago, the radiator light would go on and off occassionally, every now and then. I kept checking the coolant levels and it's full. The float goes all the way to max - maybe too full?
Anyway after some searching I figured it was the coolant level sensor. I haven't gotten around to replacing it yet. I kept a close eye on the temp gauge and it's always on the halfway point.
Today I was sitting in the car in a car park reading something. I suddenly noticed the temp gauge was at red! Horror. I switched the engine off immediately and popped the hood, but the temperature seemed normal. Will an overheating engine be ... very hot? There was no smoke or anything.
I switched it on again a minute or two later and the temp needle behaved a bit oddly. It went to normal, then slowly crept up to just under the red bar, but when I started moving slowly (stop/start traffic), it went back to the middle and stayed there for the rest of the 20 minute drive home.
I switched to the temp reading (test 7) on the dash display and the temp moved around between 90ish to 111ish. 111 was the highest I saw. It changes pretty quickly. One second it could be 93 and 4 seconds later it could be 101.
At home I waited till the engine cooled down and had a closer look. There's white residue on the thermostat connector. Coolant level is still full, engine oil level is correct. There's no milky residue, and I don't see any oil in the coolant. I'm not sure how to check for coolant in the oil though.
My question is, could it just be the thermostat (and coolant level sensor) that's the problem? Looking at how quickly the temp can change, could it be a faulty temperature sensor? I'm wondering if the brief red period really was the engine overheating.
Like you stated, my engine temp will start off in the middle, then creep up slowly to almost the red. Then sometimes at stop signs/lights, it will slowly creep down, then sometimes creep all the way up to the red. That's when I pull over and turn off the engine. I then check the engine bay and nothing is too hot. My coolant level's fine, and after a few minutes, my engine temp gauge is back down to 1/4 or 1/2. Anyways, I hope it's not too dangerous, but like I said, I'm going to attempt driving it to the garage.